Article pubs.acs.org/EF
Characterization of Coke Deposition in the Catalytic Fast Pyrolysis of Biomass Derivates Huiyan Zhang, Shanshan Shao, Rui Xiao,* Dekui Shen, and Jimin Zeng Key Laboratory of Energy Thermal Conversion and Control, Ministry of Education, Southeast University, Nanjing 210096, People’s Republic of China ABSTRACT: Coke deposition on the zeolite catalysts in the conversion of furan (a main intermediate of biomass fast pyrolysis) is of serious concern for catalyst deactivation and product distribution. It is important to find out the nature and composition of coke on the spent ZSM-5 catalyst to study the coke-depositing behaviors. In this work, spent ZSM-5 catalysts obtained from furan catalytic conversion for chemicals at different reaction times and pyrolysis temperatures were characterized. The spent catalysts were first treated with hydrofluoric acid, and then the organics were extracted with CH2Cl2. The characterization of the origin coke and the treated insoluble coke were analyzed by the combination of some analytical techniques, including Fourier transform infrared spectroscopy (FTIR), high-performance liquid chromatography (HPLC), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and thermogravimetric analysis (TGA). The extracted organics were analyzed by HPLC to determine the chemical composition of the soluble coke. The results show that coke formation mainly involves condensation and rearrangement steps at a low reaction temperature (200 °C). In the FTIR analysis, high aromaticity of coke species was obtained with increasing temperature, which indicates that the pyrolysis temperature plays a dominant role in the coke formation. TGA reveals that high temperature favors the formation of hard coke. The results enhance the understanding of coke formation and adjusting mechanism in biomass catalytic pyrolysis process.
1. INTRODUCTION Recently, the rapidly increasing need for renewable fuels is stimulating us to seek scientific and economic techniques to obtain them. Bioenergy has been regarded as a promising alternative energy.1,2 Thermochemical conversion of biomass is typically achieved by one of three processes: combustion, gasification, and pyrolysis. Pyrolysis is an important route of biomass use. Catalytic fast pyrolysis (CFP) of biomass is a promising technology for chemical production and will make great contributions to the energy supply in the future.3 Catalytic pyrolysis produces the primary product that can be stored and transported more easily than the foundation biomass material. The liquid product is called bio-oil, which can be used in several ways, including being upgraded to transportation fuels and chemicals or being directly used as fuels in boilers and gasifiers. Zeolites have been frequently used in CFP of biomass and bio-oil upgrading. However, they are deactivated rapidly because of coke formation or acid site poisoning.4,5 It is found that the activity loss is associated with the coke deposition on the zeolites. Up to now, the mechanism of coke formation is not completely clear. It is important to understand the nature of coke in the catalyst, which can help us to find methods to reduce coke formation and improves the efficiency of catalyst regeneration. The nature, chemical compositions, and locations are determined by the operation conditions [temperature, time on stream (TOS), weight hourly space velocity (WHSV), and partial pressure] and the features of the catalyst (number, strength, and distribution of the active sites and size and shape of pores and openings).6 The pore size is expected to be large enough to allow for the desired target reaction steps and narrow enough to limit the coke formation. © XXXX American Chemical Society
Appropriate operating parameters are required to limit the formation of coke precursor and its desorption from the catalyst. Guisnet et al. reported that the reagent can result in the great difference of coke species, while the coke formation depends closely upon the pore structure and size limitation of the catalyst at a high temperature.7 Deactivation of zeolite on various reagents by coke formation has been extensively investigated in the last 20 years. Most of them focused on the characterization of coke (the structure, chemical nature, location, etc.) and the kinetic study of coke deposition and regeneration. Analytical techniques lead to the information on elemental composition, physical morphology [scanning electron microscopy (SEM) and transmission electron microscopy (TEM)], chemical structure of coke components [spectroscopic techniques, such as Fourier transform infrared spectroscopy (FTIR) and solid 13C crosspolarization magic angle spinning nuclear magnetic resonance (CP/MAS NMR)], and chemical composition [chromatograph techniques, such as gas chromatography/mass spectrometry (GC/MS), gas chromatography−flame ionization detector/ thermal conductivity detector (GC−FID/TCD), high-performance liquid chromatography (HPLC), etc.].8−11 Recently, researchers focused more on the kinetics because of the limitation of analytical techniques.12 On the basis of the analysis of data, Voorhies built a typical equation that related the coke Special Issue: 4th (2013) Sino-Australian Symposium on Advanced Coal and Biomass Utilisation Technologies Received: July 28, 2013 Revised: September 17, 2013
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purity helium was used as the carrier gas. The temperature ramp of the oven temperature was set as follows: hold at 35 °C for 5 min, ramp to 140 °C at 5 °C/min, ramp to 230 °C at 50 °C/min, and hold at 230 °C for 8.2 min. Liquid products were analyzed with GC/MS. A HP-5 capillary column (30 m × 0.25 mm × 0.25 μm) was used as the chromatographic column, and high-purity helium was used as the carrier. The following temperature ramp was employed in the liquid analysis: from 40 to 50 °C at 5 °C/min, hold at 50 °C for 3 min, ramp to 80 °C at 5 °C/min, ramp to 280 °C at 10 °C/min, and hold at 280 °C for 2 min. Soluble coke of spent catalysts with different reaction times was analyzed by HPLC (Agilent 1100, Santa Clara, CA) coupling with an ultraviolet (UV) detector using a C18 column (250 × 4.6 mm). The injection volume was 20 μL. The mobile phase was prepared by mixing acetonitrile and water in 80:20 ratios, filtered, and degassed.
content with TOS. Considering that it is a semi-empirical equation, the generation mechanism of coke was introduced by Froment to form types of coke formation mechanisms, which correlated the coking rate to the concentration of coke precursors and inactivation factor. A lot of attention has been paid to the kinetic study of coke formation and catalyst regeneration in various processes.13−16 However, rarely works have been published in relation to the chemistry and structure of coke in the spent catalyst during CFP of biomass. In this work, the characterization of coke deposited on the ZSM-5 catalyst during the biomass derivate pyrolysis was performed. As an important intermediate of biomass fast pyrolysis, furan was used as the feedstock. The pyrolysis temperature was changed to study its effect on the coke characteristics, and the reaction was stopped at different TOS to understand the coking mechanism by several characterization techniques [SEM, thermogravimetric analysis (TGA), FTIR, HPLC, etc.]
3. RESULTS 3.1. Coke Formation and Its Effect on the Shape Selectivity. The aromatics and further dehydrogenated hydrocarbons formed in the cavities cannot diffuse out of the cavities, which results in the coke formation inside.19 At the early time of the reaction, a higher furan conversion on the zeolites resulted in a higher coking rate. The coking rate decreased sharply from 2.3 to 0.3% at the initial reaction period. At the late stage, a lower furan conversion led to the lower coking rate, as shown in Figure 1. The coking rate seems to be related to the furan conversion.
2. EXPERIMENTAL SECTION 2.1. Materials and Methods. Furan (>99.9%, analytical grade) from Aladdin Company, China, was used as the feedstock without any treatment. Hydrofluoric acid (HF) solution (>40%) and dichloromethane (DCM) (>99.5%) for extraction of coke were also purchased from Aladdin Company. The catalyst used in this study was ZSM-5 (CBV 3024E; SiO2/Al2O3 = 30; 50−200 mesh) supplied by Zeolyst Company. The catalytic conversion of furan was carried on a fixed-bed microcatalytic reactor, which was described eslewhere.17 For a typical run, 250 mg of catalyst was loaded into the reactor and supported by the quartz beads. Prior to the reaction, the catalysts were calcined at 600 °C in a 100 mL/min oxygen atmosphere for 6 h. The furan vapor was carried into the reactor with 20 mL/min nitrogen flow. The reaction temperature varied from 100 to 600 °C; meanwhile, the WHSV of furan was 9.08 h−1, and the partial pressure of furan was 17.78 Torr. At the specified 600 °C, the conversion was stopped to extract and identify the organics restricted in the catalysts. 2.2. Extraction of Coke from Spent Catalysts. To determine the compounds retained inside ZSM-5 during furan conversion, we used the method introduced by Guisnet and co-workers.18 After the specified reaction time ranging from 1 to 30 min, the catalysts were quenched to ambient temperature. Approximately, 100 mg of spent catalysts was first treated with a 10 mL 40% HF solution for 12 h in a Teflon vessel. We extracted the organics by adding 3 mL of DCM to the mixtures and concentrated to 1 mL. The extracted liquid presented blackish green from 100 to 400 °C and turned yellow at a higher temperature. Extracts were subjected to HPLC and FTIR. 2.3. Characterization. SEM images of the catalysts were acquired using a 1530VP scanning electron microscope (LEO, Germany). TEM pictures of insoluble coke were obtained using a JEM-2100 (JEOL, Japan). A thermogravimetric analyzer (Setsys-1750, Setaram) was used to determine the coke content of the spent catalyst. The amount of coke deposited on the catalysts was determined by the weight change during oxidization. The burning of coke was carried out in a 70 mL/ min oxygen atmosphere. Typically, 10 mg of sample was placed in the alumina crucible and heated from ambient temperature to the final temperature of 900 °C at a rate of 15 °C/min. The spent catalysts and extracted organics were scanned by a FTIR spectrophotometer (Bruker Vector 22, Germany) at room temperature. The soluble coke was first dropped on the KBr pellet and volatilized DCM. FTIR was recorded from 4000 to 400 cm−1. The products were analyzed with GC/MS (Agilent 7890A-5975C, Santa Clara, CA) to determine the chemical compositions and then qualified by GC−FID/TCD (Shimadzu 2014 GC, Japan). A Restek Rtx-VMS capillary column was used to qualify olefins, while a TDX-01 packed column was used to analyze methane, CO, and CO2. High-
Figure 1. Furan conversion and coking rate as a function of TOS.
Coke formation influenced not only the catalyst activity but also the product distribution. Figure 2 presents the detailed product yield versus TOS. It is of great interest that the yield of olefins and aromatic hydrocarbons increased at the first 2 min, which is usually called the “induction period”, and decreased at the middle of the reaction for the decrease of furan conversion.20 At longer TOS, the generation of oxygenates (methylphenol and benzofuran) resulted in the higher yield of aromatics. Chen et al. proposed that the product distribution variance caused by the coke deactivation can be a result of changes in the furan conversion, acid strength, and number and shape selectivity. In this study, the coke is supposed to be a dominant factor, known as transition-state selectivity. The selectivity in the zeolite should depend upon the void cavity, which is determined by the coke content. Figure 3 clearly shows the product distribution as a function of TOS. The mechanism of ethylene and propylene formation differs from that of butylene B
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Figure 2. Olefin and aromatic yields as a function of TOS.
and C5. With the increase of the coke amount, furan conversion decreased, while the selectivity to aromatic oxygenates increased sharply. The coke deposition cannot hinder the aromatic oxygenate formation. 3.2. SEM and TEM Imaging. The morphology of the coke on the spent catalyst was further studied by SEM and TEM shown in Figures 4 and 5, respectively. The catalysts were brick-red in color at the temperature lower than 200 °C and turned dark gray at the moderate temperature. When the temperature reached 500 °C, the catalysts looked totally black, which indicated the serious deactivation. Therefore, SEM and TEM images can be related to the absorbate or coke, which was formed during the catalytic conversion of biomass derivates. Figure 4 shows the SEM images of deactivated catalysts, which were operated at the temperature range from 100 to 600 °C. SEM images of catalysts at 200 °C revealed some stacked features, and a flat plate can be found. At 300 °C, the vogue region showed the existence of carbon-rich large molecules. With the increase of the temperature, the catalyst structure tended to be loose when compared to the fresh catalysts. The SEM photograph also presented the severe particle aggregations at higher temperatures. TEM images in the reaction time of 15 min for Figure 5 show that the alumina formed the rectangular platelets, especially at late reaction time.10 It also revealed the difference of deactivated ZSM-5 catalysts on the crystallite size. Microcrystalline ZSM-5 had the largest crystal
Figure 4. SEM photograph of the spent catalyst as a function of the pyrolysis temperature.
Figure 5. TEM photographs of the spent catalyst at different reaction times.
Figure 3. Selectivity of the product versus TOS: (a) olefins and (b) aromatics. C
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size, about 550 nm in length and 150 nm in width. The unambiguous boundary in TEM photograph after the reaction time of 30 min confirmed the idea in the SEM image. 3.3. FTIR Characterization. Figure 6 shows the FTIR spectra of the ZSM-5 catalyst for furan catalytic conversion
Figure 7. FTIR spectra of soluble coke at different reaction temperatures: (a) 100 °C, (b) 200 °C, (c) 300 °C, (d) 400 °C, (e) 500 °C, and (f) 600 °C.
3.4. Analysis of Soluble Organic HPLC. Numbers of high-carbon molecules in the CH2Cl2-soluble coke were identified by HPLC. Figure 8 presents the coke species identified at 1, 6, and 15 min with the identical scale of the Y column at the pyrolysis temperature of 600 °C. We suppose that the composition in the soluble coke includes fluorene, phenanthrene, anthracene, fluoranthene, triphenylene, and pyrene. The standard sample with 16 fractions of polyaromatic (200 μg/min) analysis proved the above idea. Generally speaking, the extraction method of soluble coke is only suitable to the coke deposition at a low temperature and for a short TOS at a high temperature.21 In the case of the zeolite catalyst, the method is universally applicable. The carbonaceous deposits are not soluble in CH2Cl2 for the huge molecules, which led to no obvious peaks in the HPLC spectra at a low temperature. It can be concluded that the inert coke species evolve toward hydrogen-poor in the biomass derivate catalytic conversion. 3.5. TGA of Spent Catalyst. TGA of the coked catalysts was carried out in pure oxygen of 70 mL/min, with the weight loss between 20 and 900 °C being considered because of the removal of the coke deposits and used to calculate the amount of coke formed on samples. Figure 9 shows the derivative thermogravimetry (DTG) curves of the coked catalyst under different pyrolysis temperatures. Because the weight loss values include removal of carbonaceous residues, including hydrogen, they are higher than the actual coke weight. Figure 8 gives four stages of mass loss: 20−165, 165−258, 258−400, and 400−600 °C; the first peak corresponds to the release of moisture and physical absorbents. It can be inferred that the coke of the catalyst is multi-composition, and the weight loss was mainly between 400 and 600 °C. The weight loss at the moderate temperature can be attributed to the soft coke, which was soluble in the organic solvent, while the major peak between 400 and 600 °C was described as “hard coke”.22,23 In Figure 9, it was found that the composition of coke converged with the increasing temperature, especially higher than 200 °C. The results indicated that a high temperature favors the hard coke formation. 3.6. Coke Formation Mechanism. Chen et al. and Guisnet reported that the organic species retained in the
Figure 6. FTIR spectra of the spent catalyst at different reaction temperatures: (a) 100 °C, (b) 200 °C, (c) 300 °C, (d) 400 °C, (e) 500 °C, and (f) 600 °C.
coked at different reaction temperatures. Several absorption bands can be found between 500 and 4000 cm−1, from which the different characteristics of the spent catalyst and soluble coke can be found. Generally, the peak at 3413 cm−1 is assigned to the associated OH. The weak C−H stretching vibration of aromatic groups can be found at 3000−3400 cm−1; meanwhile, the bands at 1530 and 1630 cm−1 represent the stretching vibrations of CC in aromatic rings. Peaks around 1400 cm−1 are attributed to the skeleton vibration of CH in CH, CH2, or CH3 in aliphatic groups. The strong absorption band at 1000− 1300 cm−1 reflects the C−O−C bending vibration from the unconverted furan. On the basis of the characteristic peaks, the bands around 1600 cm−1 are chosen as the sign of the aromatic features, while the peaks at 2900−2970 cm−1 are chosen to represent the aliphatic groups. In Figure 6, the increasing strength of peaks around 1600 cm−1 indicates that the coke contains kinds of aromatics. No peaks can be found at 3000−3400 cm−1, which revealed that the hydrogen content in the aromatic ring was little. It can be concluded that the aromatization of the coke deposited on the catalyst was attributed to the increasing temperature. The strength of acid OH peaks at 3413 cm−1 drops sharply when the temperature went up from 200 to 300 °C and then kept a constant value. Guisnet et al. infer that the low-temperature coke is formed by steps of rearrangement and condensation, while the co-oligomers and polymers in low-temperature coke are transformed into hydrogen-poor aromatic species via hydrogen-transfer steps with an increasing reaction temperature.7 Figure 7 presents the FTIR spectra of soluble coke at different reaction temperatures. The FTIR spectrogram of soluble coke can be classified into two regions, 100−200 and 300−600 °C, respectively. The soluble coke structure at low temperature is quite different from that at higher temperatures (>200 °C). Thus, the reaction temperature plays a determination role on the coke formation.21 D
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species were formed. Kolboe and co-workers have studied the nature and amount of retained material within the catalysts Hbeta, SAPO-34, and H-ZSM-5.28,29 It was revealed that the hydrocarbon pool formation is a reactant consumption process. Once the pool was formed, olefins and aromatics desorped and released from the catalysts. Nevertheless, the active organic species can be converted into larger aromatic molecules deposited in the pores or on the catalyst surface and reduce the catalyst activity. At low temperatures, the formation of carbonaceous deposits involves mainly condensation and rearrangement steps. Therefore, these deposits are generally oligomers and polymers mainly from the reactant. On the opposite side, the composition of hightemperature coke is generally polyaromatics; hence, their formation involves not only condensation and rearrangement steps but also various hydrogen-transfer steps and dehydrogenation steps on acidic catalysts.30,31 The possible mechanism of active and inert coke formation is shown in Figure 10.
Figure 10. Possible mechanism of coke formation in the furan catalytic conversion.
4. CONCLUSION The character of coke deposited on the spent catalysts for pyrolysis of furan has been investigated in this study. The results show that the pyrolysis operation (such as temperature and TOS) affects the coke deposition on the catalysts dramatically. FTIR and HPLC analyses reveal that the coke components (mainly formed by hydrogenation and hydrogen transfer) are mainly polyaromatics at higher temperatures (>200 °C), while the coke is aliphatic compounds (mainly formed by condensation and rearrangement steps) at lower temperatures. Four mass-loss stages were observed for the coke regeneration using the TGA method. The peaks at higher temperature corresponded to the combustion of soft coke and hard coke at moderate (165−400 °C) and high (400−600 °C) temperatures, respectively. TGA reveals the peak evolution toward hard coke with the increasing pyrolysis temperature.
Figure 8. Aromatic hydrocarbons detected by HPLC in the soluble coke extracted from the spent catalyst at different reaction times: (a) 1 min, (b) 6 min, and (c) 15 min.
zeolite played determining roles in the MTO process, which acted as active sites in the early stage and then inhibited the catalytic activity after that point.24 In the induction period, the basic premise for the proposed machanism is the hydrocarbon pool model, which suggests that the actual active sites in the zeolite are organic−inorganic hybrids consisting of cyclic organic species contained with the zeolitic framework.25−27 It is of great interest that the olefin and aromatic yields did not keep high at the initial reaction, in which the active organic
Figure 9. TGA profiles of the ZSM-5 catalyst under different pyrolysis temperatures: (a) 100 °C, (b) 200 °C, and (c) 500 °C. E
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(28) Olsbye, U.; Bjørgen, M.; Svelle, S.; Lillerud, K. P.; Kolboe, S. Catal. Today 2005, 106 (1−4), 108−111. (29) Svelle, S.; Rønning, P. O.; Kolboe, S. J. Catal. 2004, 224 (1), 115−123. (30) Wang, Y.; Mourant, D.; Hu, X.; Zhuang, S.; Lievens, C.; Li, C. Z. Fuel 2013, 108, 439−444. (31) Magnoux, P.; Cerqueira, H. S.; Guisnet, M. Appl. Catal., A 2002, 235 (1−2), 93−99.
This study consolidates the understanding of coke formation and will promote the catalyst regeneration during the CFP of biomass.
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AUTHOR INFORMATION
Corresponding Author
*Telephone: +86-25-83795726. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work is financially supported by the National Natural Science Foundation of China (Grants 51076031 and 51306036) and the National Basic Research Program of China (973 Program) (Grant 2010CB732206).
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